The present invention relates to the recovery of desired hydrocarbons, preferably olefins, from cat-cracked hydrocarbon gas streams. More particularly, the invention relates to the recovery of olefins from cat-cracked gas streams while avoiding the accumulation of unwanted oxides of nitrogen and their reaction products, such as nitric oxide, nitrogen dioxide, dinitrogen trioxide, nitro gums, ammonium nitrite and ammonium nitrate. Accumulations of these compounds have been observed in ethylene recovery facilities. Such accumulations can cause various operating problems, such as equipment plugging and explosion hazards.
Typically, olefins are recovered from cat-cracked gases using cryogenic fractionation in which the coldest temperatures normally fall well below -106.67.degree. C. (-160.degree. F.), and may dip as low as -167.78.degree. C. (-270.degree. F.). Unfortunately, cat-cracked gases tend to be contaminated with nitrogen oxides. Nitric oxide (NO) is of concern in cryogenic separation facilities because nitric oxide boils at a temperature close to the boiling point of methane. Thus, nitric oxide tends to follow the lighter compounds contained in the refinery gas stream. At the very low temperatures used during cryogenic fractionation, nitric oxide may be oxidized by oxygen, which typically is present in cat-cracked gases, to form unwanted nitrogen dioxide (NO.sub.2) and dinitrogen trioxide (N.sub.2 O.sub.3). If ammonia is present during the cryogenic fractionation process, ammonium nitrite (NH.sub.4 NO.sub.2) and ammonium nitrate (NH.sub.4 NO.sub.3) may be formed. In the presence of unsaturated hydrocarbons, nitrogen oxides also can react to form NO.sub.x gums.
Nitric oxide and nitrogen dioxide are poisonous gases which are undesirable for obvious reasons. Ammonium nitrite, ammonium nitrate, dinitrogen trioxide, nitrogen dioxide and NO.sub.x gums solidify at the extremely low temperatures used during cryogenic fractionation, and, as a result, may plug the equipment and/or may cause a pressure drop in the system. Ammonium nitrite also has been known to decompose spontaneously at temperatures of around 60.degree. C. (140.degree. F.), while ammonium nitrate is reported to decompose spontaneously at 210.degree. C. (410.degree. F.). NO.sub.x gums, particularly those NO.sub.x compounds formed with diolefins, such as butadiene, are reported to be unstable and to explode spontaneously at various temperatures. For all of these reasons, researchers have tried to develop methods to refine cat-cracked gases without accumulating these unwanted nitrogen-based byproducts.
A number of processes have been developed for removing nitrogen based substances from equipment used to refine gases containing oxides of nitrogen. These processes typically are costly and burdensome because they require that the process be shut down so that the equipment involved can be washed or otherwise treated to remove accumulations of the undesirable compounds. Few, if any, preventative processes have been developed by which cat-cracked gas may be refined without accumulating the undesired compounds in the first place. A preventative process which would avoid the accumulation of these compounds would be highly desirable.